Embodiments of the present invention relate to bus protocols for devices in computing systems. In particular, the embodiments relate to bus protocols that use a command-response protocol to manage transactions.
Modern computer systems include a plurality of integrated circuit devices interconnected with other devices via computer busses. Bus architectures vary considerably among these systems and many systems, in fact, include multiples of busses having different bus architectures. The different bus architectures can include different protocols used to manage the flow of data among agents connected to the bus. For example, one bus protocol may employ a command-response protocol in which a source agent on the bus transfers a packet of data to a destination agent according to a “command” transaction posted on the bus. The “command” transaction typically includes the data and also includes an indication of what is to be done with the data. The destination agent may generate a “response” transaction indicating either that the packet was received properly or that the commanded action has been completed. The command-response protocol has been used on busses to which more than two agents are connected.
These modern systems can be “bus-limited.” Conventionally, the rate at which agents can process data far exceeds the rate at which the busses between them can carry data. Thus, the speed of the bus can limit the performance of a computer system. These performance limits are particularly acute in systems where several agents are coupled to a common bus, each of them are faster than the bus and each of them compete against the other agents for bus resources.
The inventor had identified a need in the art to conserve resources of busses that operate according to the command-response protocol. In particular, the inventor recognized a need to limit the number of response transactions that are posted on the bus. These response transactions have limited utility—while they permit the source and destination agents to synchronize their operations, they do not transfer data packets themselves.